1. Field of the Invention
The present invention relates to an antenna, more particularly relates to an antenna wherein an antenna part to which a feeder circuit is connected is simple in configuration and which can be used for an antenna for transmission and reception of waves from radar.
2. Description of the Related Art
In the past, as one art for improving the driving safety of automobiles and other vehicles, there has been on-board radar used for prevention of collisions and for adaptive cruise control. On-board radar transmits waves in the front direction from a vehicle and receive waves reflected at a target object (physical markers) positioned in the front direction from the vehicle so as to estimate the distance and angle between the vehicle and physical markers. In such radar, a microstrip patch antenna or slot antenna is used for transmission/reception of waves.
A general microstrip antenna is provided with a dielectric substrate, a patch antenna part which is formed on the dielectric substrate by etching, and a ground plate which is formed on the bottom surface of the dielectric substrate. The patch antenna part and the ground plate are formed by copper foil. The ground plate is also called a “grounding plate” or “earthing plate”. Further, the patch antenna part has a feeder circuit connected to it.
A microstrip patch antenna provided with a patch antenna part and a feeder circuit connected to it is formed with slits in the patch antenna part for impedance matching. A feeder line matched to an input impedance of 50Ω is connected to the patch antenna part. The length of the patch antenna part in the polarization direction is a whole multiple of the length of about half of the wavelength by which the operating frequency of the wave transmitted or received is propagated (hereinafter referred to as the “guide wavelength”).
Further, in another conventional example of a microstrip patch antenna provided with a patch antenna part and a feeder circuit connected to it, an impedance transformer is formed at the feeder circuit for enabling connection to the patch antenna part at a high impedance end. The feeder circuit which is connected to the impedance transformer makes the input impedance match 50Ω. The length of the patch antenna part in the polarization direction is a length of a whole multiple of about half of the guide wavelength, while the length of the impedance transformer is a whole multiple of one-quarter the length of the guide wavelength.
On the other hand, as the transmission/reception antenna of on-board radar, use of a flat array antenna using microstrip conductors is disclosed in Japanese Patent No. 3306592. Further, a slot array antenna comprised of a ground plate in which slot lines are provided and at the two sides of the slot lines of which slot devices are formed is disclosed in Japanese Patent Publication (A) No. 2001-111337. The flat array antenna disclosed in Japanese Patent No. 3306592 transmits and receives polarized waves in a direction inclined from the microstrip line. Japanese Patent No. 3306592, FIG. 7(b), discloses an example in which the terminal end of the feeder strip line is made to effectively radiate power by providing a microstrip antenna device provided with a patch antenna path formed with slits. Similarly, the slot array antenna disclosed in Japanese Patent Publication (A) No. 2001-111337 transmits and receives polarized waves in a direction inclined from the slot line. Japanese Patent Publication (A) No. 2001-111337, FIG. 7(b), discloses an example of provision of a slot element for effectively radiating power from the terminal end of the slot line.
However, if providing slits in the patch antenna part for impedance matching between the patch antenna part and the feeder circuit, the antenna shape becomes complicated and etching of the patch antenna part becomes difficult. Further, if connecting an impedance transformer of a high impedance to the patch antenna part, the width of the line of the impedance transformer becomes extremely fine. The line width ends up becoming narrower than the minimum line width of the processing limit. Processing therefore cannot be guaranteed.
Further, when using a microstrip patch antenna for high frequency transmission/reception, the wavelength of the operating frequency is short, so a small dimensional error will have a large effect on performance. That is, a conventional structure of a microstrip patch antenna has a large number of end faces and a complicated structure, so there was the problem of a large deterioration in performance due to manufacturing error at the time of pattern formation by etching etc. Further, in the slot array antenna disclosed in Japanese Patent Publication (A) No. 2001-111337 as well, there is a similar problem as with microstrip patch antenna of deterioration of the performance due to manufacturing error at the time of processing to form the slot patterns of the slot antenna. Further, as disclosed in Japanese Patent Publication (A) No. 2001-111337, FIG. 7(b), when connecting a corner of a slot element to the terminal end of a slot line, there was the problem that the residual power reaching the terminal end was not effectively radiated. Note that the above Japanese Patent No. 3306592 (Japanese Patent Application No. 2000-54606) and Japanese Patent Publication (A) No. 2001-111337 (Japanese Patent Application No. 11-141170) were combined for filing in the U.S. and have been granted as U.S. Pat. No. 6,424,298B1.